Murmurhash 哈希算法

Murmurhash 哈希算法

MurmurHash 是一种非加密型哈希函数，适用于一般的哈希检索操作。 由Austin Appleby在2008年发明， 并出现了多个变种，都已经发布到了公有领域(public domain)。与其它流行的哈希函数相比，对于规律性较强的key，MurmurHash的随机分布特征表现更良好。

Redis在实现字典时用到了两种不同的哈希算法，MurmurHash便是其中一种（另一种是djb），在Redis中应用十分广泛，包括数据库、集群、哈希键、阻塞操作等功能都用到了这个算法。发明算法的作者被邀到google工作，该算法最新版本是MurmurHash3，基于MurmurHash2改进了一些小瑕疵，使得速度更快，实现了32位（低延时）、128位HashKey，尤其对大块的数据，具有较高的平衡性与低碰撞率。

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unit Grijjy.Hash;   {$INCLUDE 'Grijjy.inc'}   {$OVERFLOWCHECKS OFF} // required since overflow checks will fail (code works ok w/o checking on)   interface   type   { Incremental Murmur-2 hash.       See https://sites.google.com/site/murmurhash/     Uses the CMurmurHash2A variant, which can be used incrementally.     The results are *not* the same as for goMurmurHash2 in Grijjy.SysUtils }   TgoHashMurmur2 = record   {$REGION 'Internal Declarations'}   private const     M = $5bd1e995;     R = 24;   private     FHash: Cardinal;     FTail: Cardinal;     FCount: Cardinal;     FSize: Cardinal;   private     class procedure Mix(var H, K: Cardinal); static; inline;   private     procedure MixTail(var AData: PByte; var ALength: Integer);   {$ENDREGION 'Internal Declarations'}   public     { Starts a new hash.         Parameters:         ASeed: (optional) seed value for the hash.         This is identical to calling Reset. }     class function Create(const ASeed: Integer = 0): TgoHashMurmur2; static; inline;       { Restarts the hash         Parameters:         ASeed: (optional) seed value for the hash.         This is identical to using Create. }     procedure Reset(const ASeed: Integer = 0); inline;       { Updates the hash with new data.         Parameters:         AData: the data to hash         ALength: the size of the data in bytes. }     procedure Update(const AData; ALength: Integer);       { Finishes the hash and returns the hash code.         Returns:         The hash code }     function Finish: Cardinal;   end;   implementation   { TgoHashMurmur2 }   class function TgoHashMurmur2.Create(const ASeed: Integer): TgoHashMurmur2; begin   Result.Reset(ASeed); end;   function TgoHashMurmur2.Finish: Cardinal; begin   Mix(FHash, FTail);   Mix(FHash, FSize);     FHash := FHash xor (FHash shr 13);   FHash := FHash * M;   FHash := FHash xor (FHash shr 15);     Result := FHash; end;   class procedure TgoHashMurmur2.Mix(var H, K: Cardinal); begin   K := K * M;   K := K xor (K shr R);   K := K * M;   H := H * M;   H := H xor K; end;   procedure TgoHashMurmur2.MixTail(var AData: PByte; var ALength: Integer); begin   while (ALength <> 0) and ((ALength < 4) or (FCount <> 0)) do   begin     FTail := FTail or (AData^ shl (FCount * 8));     Inc(AData);     Inc(FCount);     Dec(ALength);       if (FCount = 4) then     begin       Mix(FHash, FTail);       FTail := 0;       FCount := 0;     end;   end; end;   procedure TgoHashMurmur2.Reset(const ASeed: Integer); begin   FHash := ASeed;   FTail := 0;   FCount := 0;   FSize := 0; end;   procedure TgoHashMurmur2.Update(const AData; ALength: Integer); var   Data: PByte;   K: Cardinal; begin   Inc(FSize, ALength);   Data := @AData;   MixTail(Data, ALength);   while (ALength >= 4) do   begin     K := PCardinal(Data)^;     Mix(FHash, K);     Inc(Data, 4);     Dec(ALength, 4);   end;   MixTail(Data, ALength); end;   end.